After years of study and consideration, this is the most elusive answer of all. Leadership is so contextual and conditional that there isn't really a model, if there were, leadership would be far easier to replicate.

We need to think through what is meant by success and, in fact, what we mean by a model of leadership in the first place. Understanding these two points will allow us to dig deeper into the broader questions our book considers in depth.

So how do we define successful leadership?

Often when we speak about leadership we have lots of implicit assumptions about the term. Success in one sense might be a military general helping his or her country win a war, a politician winning an election, or a CEO delivering record-breaking profits.

But in fact we rarely remember leaders as success stories using such demonstrable or quantifiable metrics. It is rare that we are so rational or dispassionate in our assessments.

In our book, we profile Robert E. Lee, showing that he became a leader of mythic proportion in the United States not because he won a war (he didn't), nor because he supported a good cause (he didn't), but because he offered something unique and irreplaceable to Confederate soldiers, and latterly to many Americans and soldiers in the reunited union.

Instead of thinking about leadership as successful or not, we believe a better way of understanding leadership is how far it is effective. Effectiveness in leadership allows us to think about leadership not as something black and white that delivers results--or not--but a practice and quality that may be in part be about delivering results, but relates as much to providing symbolism, meaning and values to a group of followers.

That, for example, allows us to understand why Robert E. Lee was so singularly effective, despite the fact he led an army and cause which ended up losing.

This also means that effective leadership is not a moral force that necessarily brings good along with it. While there are stand-out cases like Martin Luther King, Jr, an argument can easily be made that Maximilien Robespierre was a highly effective leader of the French Revolution, even though his name has become inseparable from the bloody Terror which killed thousands of people, and the gains he made were washed away by counter-revolution and then a return to monarchy (both of these leaders are profiled in our book).

Do these leaders offer us models to aspire to? We hope, in the cases of Robert E. Lee and Maximilien Robespierre, that readers will be able to recognize the palpably immoral aspects to their actions and legacies, even if they can draw from them some measure of historical appreciation and nuance.

Onto models. Usually when we think about models of leadership, we're not talking about following individuals, but philosophical and practical approaches. Historically, leaders read Plutarch (on whose writings our book is modeled) or Machiavelli as a way of learning what it takes to be a great leader. Today, instead we often hear of theories like "Authentic Leadership" and "Charismatic Leadership" as styles which can be effective.

While we can learn a lot from reading about these models of leadership, rare is it that a leader has the autonomy, or the capacity, to act in exactly the way such leadership theories suggest we should. In fact, what we argue in our book is that there is a Formulaic Myth at work.

This means we're constantly trying to create a checklist or set of practices that together make for effective leadership. What we argue is that each moment of leadership calls for something unique and different. For truly effective leadership, formulas just won't cut it.

For example, it's undeniable that Margaret Thatcher (whom we profile) was Britain's most effective prime minister since Winston Churchill. But when one thinks of Mrs Thatcher, she didn't have bucket loads of the qualities which leadership theorists tell us are required for effective leadership today, like empathy and good team-building skills. A lot of the time, she did the opposite of what a modern leadership guru would tell you to do. Her legacy as the Iron Lady proves that.

What we can conclude here is that the most effective leaders understand in a singularly insightful way what is required of them in the moment -- the unique context of that situation -- and then they act on it. Sometimes that produces results congruent with what's best in us, like with Martin Luther King. Other times, it does the opposite -- and today, unfortunately, there are countless examples of that. This is why it's imperative for people to question what values their leaders represent, for to some extent it is a mirror that reflects back on themselves.

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